WEIGHT OF ATMOSPHERIC COLUMN—TENSION OF VAPOR. 
55 
The altitude of each camp is determined from a series of observations with mercurial barome¬ 
ters. The profile of each day’s march is deduced from single observations, made with mercurial 
or aneroid barometers, at points where decided changes of elevation or depression occurred. The 
aneroid, when in use, was compared with the mercurial at every third hour during the day ; 
from which comparison its error was determined. 
The formula for the reduction of the observations have been strictly applied in accordance 
with the established rules. These are believed to have produced satisfactory results, excepting 
upon certain points, which will be stated in the course of these remarks. The first relates to a 
constant difference between the normal condition of the atmosphere upon the sea-coast and in 
the interior of the continent, with regard to the force of vapor. By a reference to the psychro- 
metric tables it will be perceived, that at Beaverstown, in the Choctaw country, where our 
hygrometric observations were commenced, the mean elastic force of the vapor contained in the 
atmosphere was, by measurement, 17.1 millimetres, equal to 0.674 of an inch. This force was 
undoubtedly exerted upon the barometer at that place, in addition to the weight of the atmo¬ 
spheric column of dry air. Upon the Rocky mountains, east, and Sierra Madre, west of the 
Rio Grande—at Camps 56 and 67—the force of vapor measured from three to three and a half 
millimetres ; or, at a mean, 0.126 of an inch. Therefore, to obtain the true differences between 
the weights of the atmospheric columns at the points referred to, it would be necessary to correct 
each barometer reading by the force of vapor known to be exerted upon it. The effect of such a 
correction, applied to our barometric observations, would be to diminish the computed altitudes, 
upon the most elevated parts of our survey, by nearly 600 feet ;* which would greatly improve 
® Upon the acknowledged principle that the vapor of water follows the same law as gases—viz: that one acts as a va¬ 
cuum to another—it seems evident that the tension of vapor in the atmosphere must be a force exerted in addition to the 
gaseous pressure ; and as the barometer measures the sum of the two forces, if either be known, it should be deducted 
from the barometric pressure to obtain the other. A distinguished meteorologist has suggested that a portion of the at¬ 
mospheric column may be removed laterally, and replaced by the vapor of water, which, being a lighter fluid, may tend 
to neutralize, by diminished weight, the effect of increased elastic force. For example : when there exist surface currents 
of wind, highly charged with moisture, the heavy stratum of dry air is then replaced by a moist and light one ; and, con¬ 
sequently, although the elastic force of vapor may have increased, the diminished pressure of the atmosphere effects a 
compensation, and the barometer does not rise. An occurrence of this kind would, I believe, form an exception to the 
general rule. At all events, it could not be applicable to a case like that in question, where a comparison is made be¬ 
tween places far distant from each other, at which the normal conditions of the atmosphere are essentially different. The 
more I have examined the subject, the stronger has become my conviction that the barometric pressure at each point 
should have been divested of the force of vapor, and the weight of dry air alone used in the determination of differences 
of altitudes. However, in deference to the high authority of doubts that have been expressed regarding its propriety, I 
have refrained from making those corrections which would have so greatly modified and improved the profile of our route. 
The following extract from a report of Lieutenant Colonel Sabine, R. A., “ on some points in the meteorology of Bom¬ 
bay,” discovered by me as this paper was about to be submitted to the department, has a direct bearing upon the subject 
under consideration : 
“ In concluding this communication, I beg respectfully to submit to the consideration of the eminent meteorologists 
here present, that it is very important towards the progress of this science that the propriety (in such discussions as the 
present) of separating the effect of the two elastic forces which are considered to unite in forming the barometric pressure, 
be speedily admitted or disproved. The very remarkable fact recently brought to our notice by Sir James Ross, as one of 
the results of his memorable voyage, that the mean height of the barometer is full an inch less in the latitude of 75° S. 
than in the tropics, and that it diminishes progressively from the tropics to the high latitudes, presses the consideration 
of this point upon our notice ; for it is either explained wholly or in greater part by the diminution of the vapor constitu¬ 
ent in the higher latitudes, which diminution appears nearly to correspond throughout to the decrease of barometric pres¬ 
sure observed by Sir Jarre's Ross ; or it is a fact unexplained, and I believe hitherto unattempted to be explained, on any 
other hypothesis, and of so startling a character as to call for immediate attention. 
“ If, by deducting the tension of the vapor from the barometric pressure, we do indeed obtain a true measure of the 
pressure of the gaseous portion of the atmosphere, the variations of the mean annual gaseous pressure, which will thus be 
obtained in different parts of the globe, and the differences of pressure in different seasons at individual stations, may be 
expected to throw a much clearer light than v y e have hitherto possessed on those great aerial currents which owe their 
origin to variations of temperature proceeding partly from the different angles of inclination at which the sun's rays are 
received, and partly from the nature and configuration of the material bodies at the surface of the earth ; and a field of 
research appears to be thus opened, by which our knowledge of both the persistent and the periodical disturbances of the 
equilibrium of the atmosphere may be greatly extended.” 
